(i) Field of the Invention
This invention relates generally to porous nonwoven fabrics formed from selfbonded heterofilaments. More particularly it relates to such fabrics in which the heterofilament has a core of isotactic polypropylene and a sheath; the sheath is high density polyethylene; and the sheath comprises from 5 to 30 weight percent of the heterofilament.
(ii) Description of the Prior Art
It is well known that batts or webs consisting solely of randomly laid thermoplastic homofilaments having essentially identical properties cannot be area-selfbonded commercially by application of heat and pressure to yield a product having both high grab strength and Elmendorf tear strength, on account of extreme criticality of the bonding conditions. For example, du Pont's U.S. Pat. No. 3,546,062 teaches an acceptable bonding temperature range of only a fraction of a degree Centigrade for polypropylene fibers, other conditions being kept constant, and that overbonding results in a cardboard like product. The prior art further shows that the operating range of permissible bonding temperature is greatly increased by deliberately introducing variability into the raw materials and/or processing conditions in many different ways. For example, U.S. Pat. No. 3,231,650, Example 1, teaches that the strength of selfbonded nonwoven webs of drawn high density polyethylene homofilaments having a softening point of 260.degree. F. may be increased by a factor of 10, as measured by manual application of tension to the selfbonded web, by soaking the drawn fibers in hydrocarbon oil for 15 minutes prior to bonding at 83.3 p.s.i. for 5 minutes at 250.degree. F. The patent does not discuss how the hydrocarbon oil causes the increase in strength, but it presumably preferentially plasticizes the amorphous portions of the highly crystalline filaments. Another method of introducing variability is that of using minor quantities of a lower-melting "binder" fiber to effectuate the desired amount of bonding. The binder fiber may be spun as separate filaments from the strength-providing fiber (as in du Pont's U.S. Pat. No. 3,546,062 and Kuraray's U.S. Pat. No. 3,914,497) or it may be co-spun with some or all of the strength-providing fiber to give heterofilaments (as in ICI U.S. Pat. Nos. 3,511,747; 3,423,266; and 3,595,731; and ICI's U.K. Patents 1,157,437 and 1,073,181; Chisso's German Offenlegungsschrift 2,358,484 and Mitsubishi Rayon's Japanese Pat. No. 50-4767; and in "Mechanical Behavior and Bonding In Nonwovens", a dissertation presented to Princeton University by C. J. Shimalla, June 1974).
The ICI patents such as U.S. Pat. Nos. 3,511,747 and 3,595,731, disclose the preamble of applicants' claimed invention, including a general disclosure of polyethylene as the potentially adhesive component of the heterofilament. However, there are no examples directed to high density polyethylene.
Prior art relating to the use of high density polyethylene in filaments includes the following.
Shimalla studied in depth the factors affecting the properties of nonwoven fabrics formed from card webs of mixtures of homofilaments and heterofilaments in which the binder was linear polyethylene in the form of a 50/50 bilateral bicomponent fiber, the conjugate half being isotactic polypropylene, and the fabrics contained up to 50 weight percent of linear polyethylene. Shimalla also points out that sheath/core filaments provide about four times as many bond points as 50/50 bilateral filaments and found that the relative distance between bonds was one of the most critical parameters in filtration applications of these nonwovens. Shimalla further analyses principles of bonding in semicrystalline polymers and points out that the bond strength is determined by a complex combination of physical, chemical and mechanical properties of the materials making up the bond, by the conditions under which the bond is formed, and by the consequent bulk and local stresses. Shimalla's requirements for practical adhesive bonding are: (1) ensure that no weak boundary layer is present on the substrate; (2) use an adhesive having a surface tension less than the critical surface tension of the substrate; (3) form extensive interfacial contacts by the choice of bonding conditions; and (4) set (cure or crystallize) the adhesive to maintain interfacial contacts, prevent frozen stresses, and eliminate weak boundary layers.
U.S. Pat. No. 3,914,497 teaches that high density polyethylene homofilaments used as 10% binder fiber for polypropylene fibers gives a bonded fabric having very low tensile strength (see U.S. Pat. No. 3,914,497 Comparative Example 8).
U.S. Pat. No. 3,620,892 discloses fabrics of fused heterofilaments consisting of minute fibrils which may be polypropylene and polyethylene. No distinction is made between high density polyethylene and low density polyethylene.
U.S. Pat. No. 3,760,046, column 7, teaches the sintering of fabrics with low thread counts in which the filaments have a low density polyethylene sheath and a core of either polypropylene or high density polyethylene.
U.S. Pat. No. 2,861,319 teaches that low density filaments containing non-continuous voids may be prepared by drawing sheath/core filaments in which the core has less extensibility than the sheath, and does not show the use of a high density polyethylene sheath with an isotactic polypropylene core.
U.S. Pat. No. 3,998,988 relates to fibrous material having particulate material studded in the surfaces of the filaments. It discloses a fibrous adsorptive material in the form of tow, web, fabric, sheet, ball or flock consisting of a sheath/core conjugate fiber of a high melting core component and a low melting sheath component, with finely divided particles of an absorbent embedded in the surface of the low melting sheath component. It further teaches that the sheath has a melting point at least 40.degree. C. lower, and preferably 50.degree. C. lower, than the melting point of the core, and polyethylene may be the sheath. Example 20 of the patent discloses fibrous beads formed by heating a tumbled mixture of 55% sheath/core drawn filaments (5 mm long with the core being polypropylene and the sheath being polyethylene having a melting point of 132.degree. C.) with 45% of particulate absorbent material, whereby the absorbing agent particles are melt adhered to filament surfaces and fixed among filaments to form a bead-like fibrous absorbent. The presence of particulate material between two frozen surfaces would be expected to significantly weaken the bond strength.
Chisso's German Offenlegungsschrift No. 2,358,484 discloses nonwoven fabrics comprising crimped bicomponent heterofilaments of polypropylene and high density polyethylene. It does not exemplify sheath/core heterofilaments, but rather rod/crescent side/side heterofilaments approaching sheath/core filaments. It states that there is no particular limitation of the mixture ratio of the two components; that a weight proportion of 40-70% of the lower melting component is preferred; and exemplifies 50-60%. It further teaches that the melt flow ratio of the polypropylene component to the high density polyethylene component should not exceed 5.0 (see Chisso's Comparative Example 2 and claim 1).
Mitsubishi Rayon's Japanese Patent No. 50-4767 relates to a process for manufacturing bicomponent fibers for synthetic fiber paper. It teaches, inter alia, that paper can be made from chopped fibers comprising heat-treated heterofilaments having a polypropylene core and a polyethylene sheath. It further teaches that low density polyethylene is most suitable as compared with high density polyethylene and medium density polyethylene. However, it does not include any actual examples relating to high density polyethylene, and all its examples relate to heterofilaments containing polyethylene having at least 50 percent of the cross-section of the filament being polyethylene (e.g. Table 1 and FIGS. 1A-1E).
Several references relate to nonwoven spunbonded film-fibril sheets of substantially continuous plexifilamentary strands of high density polyethylene. U.S. Pat. No. 3,619,339 discloses a point-bonded product having around 1.3 oz/yd.sup.2, tongue tear strength of 2.9 lb and tensile strength of 50 lb/4 in. Similarly an article entitled "Spun bonded sheet products" by Hentschel in CHEMTECH, January, 1974, pages 32-41 gives detailed properties of both area-bonded and point-bonded Tyvek.RTM. sheets. Sheets of weight 1.0-2.7 oz/yd.sup.2 have Elmendorf tear strengths of 0.8-3.5 lb, with area-bonded fabrics having Elmendorf tear strength of up to 1.1 lb. "The Properties and Processing of TYVEK.RTM. Spunbonded Olefin" Bulletin S10, published by du Pont in December, 1973 states at page 7 that "While it is possible to fuse TYVEK to itself using heat only, strong seals are difficult to obtain in this way: melting the TYVEK destroys the fiber structure, reducing both the flexibility and tear strength in the seal area. The preferred method is to apply a coating whose melting point is below that of TYVEK, such as branched polyethylene or SURLYN A. With such a coating, high seal strengths can be achieved using hot bar or impulse techniques."
The foregoing prior art is shown diagrammatically in FIG. 1 with regard to the two parameters "amount of binder in binding filament" and "weight percent of binding filament in fabric" (see definition of "binding filament" below). Other parameters such as type of heterofilament (sheath/core or side/side) and types of polymer in the heterofilaments are given in the key to FIG. 1.